There has been research and development of techniques that form a microstructured irregularities on a material surface. Formation of such a surface microstructure enables improving bonding to the overlying film, and adhesion for liquid, or altering the optical characteristics of the surface.
Zirconia-based ceramics, a material processed in this patent application, are known to have high-strength, and show characteristics that vary with the concentration and the type of additive. A material doped with a predetermined concentration of yttria (yttrium oxide) has toughness, and applications to biomaterials and machine materials are expected. This type of zirconia-based ceramic has also been used for oxygen sensors by taking advantage of the oxygen ion conducting property of the material.
Zirconia-based ceramics are not easily machineable because of mechanical characteristics such as toughness, hardness, and abrasion resistance. However, despite the poor workability, these desirable mechanical characteristics of zirconia-based ceramics have been exploited in biomaterial applications such as dental implants, and substitute bone. However, zirconia-based ceramics themselves have low compatibility to the body, and a technique that coats a highly biocompatible material such as apatite over the material surface is desired to improve biocompatibility. Materials containing tetragonal zirconia have desirable mechanical characteristics with no toxicity, and have been widely used as materials for medical equipment. Currently, these materials are used in a variety of practical applications, including artificial joints, dental restorations such as crowns, and artificial roots. However, tetragonal zirconia is inferior to calcium phosphate in terms of tissue conductivity, a property that encourages conduction of natural normal tissue to a material surface at the placement site. Because of this property, a thin fibrous connective tissue (with a thickness of 1 to 10 μm) occurs on a surface of the tetragonal zirconia placed in bone or soft tissue, and the material does not directly connect itself to the bone or soft tissue. It is also worth mentioning that, unlike calcium phosphate, there is no report of tetragonal zirconia in relation to blood compatibility. Accordingly, it would be useful to have a method that coats a surface of a ceramic material such as zirconia with a tissue conductive material to allow the material to directly connect itself to the bone in the body after operation. Calcium phosphates such as hydroxyapatite (a bone component) are considered to be most suited for the purpose of connecting a ceramic material to bone, and it has been proposed to coat a surface of a metallic material or a ceramic material such as zirconia with calcium phosphate to this end.
A prior art search for a method that improves adhesion for a film through formation of a microstructure on a surface of zirconia and other materials found the following techniques.
PTL 1 reports a nanolevel microprocessing technique whereby an ultrashort pulsed-laser (a femtosecond laser) is applied under polarization control to form a microstructure of a size smaller than the wavelength of the applied laser. A suitable fluence range is described as being from the ablation threshold to 10 times the ablation threshold. In this publication, “fluence” is described as energy per unit area (J/cm2) determined by dividing the output energy per laser beam pulse by the cross sectional area of irradiation, and “ablation threshold” is described as the minimum value of energy density at which vaporization occurs on a material surface irradiated with a laser beam. PTL 1 reports that irradiation of a solid material surface with a linearly polarized ultrashort pulsed laser beam forms a narrow projecting microstructure along a direction orthogonal to the direction of polarization, and that irradiation of circularly polarized light forms a granularly projecting microstructure. The size of the microstructure has a positive correlation with the wavelength of the applied laser, and the microstructure has a size much smaller than the wavelength ( 1/10 to ⅗ of the wavelength). As examples of surface microprocessing, PTL 1 uses a nitride ceramic (TiN) film, an amorphous carbon film, and a stainless steel material as samples.
PTL 2 reports a surface processing method that applies a single laser beam to a material surface to periodically form fine irregularities. In this patent, regions irradiated with a laser beam are scanned with an overlap, and ablation occurring at the interfering region of the p-polarized component of the incident light and the scattered light of the p-polarized component of along the material surface forms a self-organizing periodic structure orthogonal to the polarization direction of incident light, at an interval that is no smaller than the λ/2 of the incident light component and no greater than the incident light λ. The laser is described as being irradiated in 10 to 300 shots at a given region. PTL 2 takes advantage of the periodic structure that is formed in a direction orthogonal to the direction of polarization of incident light, and the direction of periodic structure is variable by varying the direction of polarization of incident light. A suitable fluence range is described as being in the vicinity of the ablation threshold. Surfaces of a Si substrate, a Cu tape, and an Al tape are examples of the samples worked in PTL 2.
PTL 3 proposes a method for fabricating a highly bone-compatible hydroxyapatite film on a silicon or stainless steel base material through vapor deposition of hydroxyapatite after fabricating periodic irregularities on a base material surface using an ultrashort pulsed-laser.
PTL 1 to PTL 3 are techniques that form periodic grooves in a direction orthogonal to the direction of polarization of incident light. There is no report of experiments conducted on zirconia-based ceramics.
In one of the few reports found by our prior art search for processing of zirconia-based ceramics with a femtosecond laser beam, NPL 1 describes direct ablation processing of dental implant zirconia on the order of several tens of micrometers, and evaluation of surface conditions after the process, and reports that the process has only a small effect on crystalline phase. There is no report of periodic structures of a submicron size.
A prior art search for the relationship between a direction of polarization of incident light and a microstructure found a report of periodic grooves formed in a SiO2 material in a direction parallel to the direction of polarization of incident light (see NPL 2).